In a general wireless access system, only a single carrier may be considered even though bandwidths between an uplink and a downlink are set up to be different from each other. For example, on the basis of a single carrier, a wireless communication system may be provided, in which the number of carriers constituting the uplink and the number of carriers constituting the downlink may be 1, respectively, and a bandwidth of the uplink is symmetrical to that of the downlink.
In the International Telecommunication Union (ITU), it is required that the candidate technology of the IMT-Advanced should support an extended bandwidth as compared with a wireless communication system according to the related art. However, except for some areas of the world, it is difficult to allocate frequencies of wide bandwidths. Therefore, as a technique for effectively using fragmented small bands, a carrier aggregation (bandwidth aggregation or spectrum aggregation) technique is being developed to obtain the same effect as when a band of a logically wide bandwidth is used by physically aggregating a plurality of bands in a frequency domain.
The carrier aggregation is introduced to support increased throughput, prevent the cost from being increased by a wideband RF device, and ensure compatibility with the existing system. The carrier aggregation refers to a technique of enabling data exchange between a user equipment and a base station through a plurality of groups of carriers of a bandwidth unit defined in the existing wireless communication system.
In this case, the carriers of a bandwidth unit defined in the existing wireless communication system may be referred to as component carriers (CC). For example, the carrier aggregation technique may include a technique for supporting a system bandwidth of maximum 100 MHz by using maximum five component carriers even if one component carrier supports a bandwidth of 5 MHz, 10 MHz or 20 MHz.
If the carrier aggregation technique is used, data may simultaneously be transmitted and received through several uplink/downlink component carriers. Accordingly, the user equipment may monitor and measure all the component carriers.